Method of controlling the direction of propagation of injection fractures in permeable formations

ABSTRACT

The invention relates to a method of controlling the production of oil or gas from a formation ( 1 ) comprising that a first and second drilled production well ( 105, 110 ) are formed next to each other that extend essentially horizontally, that, at the drilled production wells, a further drilled well ( 115 ) is formed that extends between the first and the second drilled production well ( 105, 110 ), that the production of oil or gas is initiated, and that, while oil or gas is being produced, a liquid is conveyed to said further drilled well ( 115 ) and out into the formation ( 1 ) for a first period of time T 1 . The invention is characterised in that the pore pressure of the formation is influenced during the period T 1  with the object of subsequently controlling the formation of fractures along a drilled well, typically across large distances in the reservoir. Such influence is accomplished partly by production in adjacent wells, partly by injection at low rate without fracturing in the well in which the fracture is to originate. Injection at low rate presupposes that an at least approximated determination is performed of the maximally allowable injection rate I max  for the period T 1  in order to avoid fracturing ruptures in said further drilled well ( 115 ) when liquid is supplied by the injection rate I for the liquid supplied to the further drilled well being kept below said maximally allowable injection rate I max  for said first period of time T 1  when the relation σ′ hole,min &lt;≦σ′ h  has been complied with.

[0001] The present invention relates to an improved method of thegeneral kind wherein, for the production of oil or gas from a formation,a first and a second drilled production well are formed next to eachother, and wherein a further drilled well, a so-called injection well,is established that extends at and between the first and the seconddrilled well, wherein—while oil or gas is being produced—a liquid isconveyed to the drilled injection well and out into the formation for aperiod of time T₁.

[0002] The invention is based on the fact that, during supply of liquidto a drilled injection well at high injection rates, fractures may occurthat propagate from the drilled injection well through those areas ofthe formation that have inherent weaknesses and/or in the direction ofthe maximal horizontal stress σ′_(H) of the formation. These fracturesare undesirable in case they mean that liquid flows away uncontrollablyfrom the drilled injection well directly into either the first or thesecond adjoining drilled production well, which would mean that theoperating conditions are not optimal. However, in general the formationof fractures has the advantage that the supplied liquid can more quicklybe conveyed into the surrounding formation across a larger vertical faceand is thus able to more rapidly displace the contents of oil or gas.

[0003] By the invention it is attempted to provide a very particularfracture that extends from a drilled injection well in order to optimisethe production of oil or gas. More specifically the present inventionaims to enable control of the propagation of such fracture in such amanner that the fracture has a controlled course and will to a wideextent extend in a vertical plane along with and coinciding with thedrilled injection well.

[0004] This is obtained by performing, in connection with the methoddescribed above, at least an approximated determination of the maximallyallowable injection rate I_(max) during the period T₁ to avoidfracturing in the drilled injection well when liquid is supplied, inthat the injection rate I for the liquid supplied to the drilledinjection well is kept below said maximally allowable injection rateI_(max) for said first period of time T₁, and in that the injection rateI is increased to a value above I_(max) following expiry of the periodof time T₁ when the relation σ′_(hole,min)<=σ′_(h) has been compliedwith. The term ‘injection rate’ as used herein in this context isintended to designate the amount of liquid, expressed as amount per timeunit, supplied to the drilled injection well.

[0005] U.S. Pat. No. 5,482,116 teaches a method of controlling thedirection of a hydraulic fracture induced from a wellbore. The methoddoes not make use of induced changes to the stress field by productionand injection before fracturing.

[0006] In the present invention, the maximally allowable injection rateI_(max) for avoiding fracturing may eg be determined or estimated by theso-called ‘step rate’ test, wherein the injection rate is increased insteps while simultaneously the pressure prevailing in the well bore ismonitored. When the curve that reflects this relation suddenly changesits slope, such change is—in accordance with current theories—construedas on-set of fracture, propagation, and the injection rate I thatproduces such fracture formation is, in the following, designatedI_(max).

[0007] As taught in claim 2 it is preferred that the drilled wells areestablished so as to extend essentially horizontally, whereby thevertical stresses of the formation contribute further to the invention.The term ‘essentially horizontally’ as used in this context is intendedto designate well bores that extend within an angle range of +/−about25° relative to the horizontal plane. It is noted that the invention mayalso be practised outside this range.

[0008] It is further preferred that, prior to establishment of the wellbores, the direction of the largest effective inherent principal stressσ′_(H) of the formation in the area of the planned location of the wellbores is estimated, and that the drilled wells extend within theinterval +/−about 25° relative to this direction.

[0009]FIG. 1 shows two drilled production wells, from which oil or gasis produced, and the orientation of the principal stresses in thesurrounding formation;

[0010]FIG. 2 shows the stresses in the formation shown in FIG. 1following six months of production,

[0011]FIG. 3 shows two drilled production wells, from which oil or gasis produced, and a drilled injection well to which liquid is supplied,and the orientation of the principal stresses in the surroundingformation;

[0012]FIG. 4 shows the stresses in the formation shown in FIG. 3following six months of production and three months of water injection;

[0013]FIG. 5 explains the constituent stress notations at the drilledinjection well;

[0014]FIG. 6 shows the development, over time, of the stressesimmediately above the drilled injection well shown in FIG. 5; and

[0015]FIG. 7 illustrates a typical relation between the pressure in theinjection well and the injection rate.

[0016] In FIG. 1 reference numerals 5, 10 designate two drilledproduction wells for the production of oil or gas from a Cretaceousformation 1. The drilled production wells 5, 10 extend in anapproximately shared plane in the formation 1 at a depth of eg about7000 ft below sea level. The shown shared plane is horizontal, but itmay have any orientation. For instance, the drilled production wells 5,10 may extend in a plane with a slope comprised within the interval +/−about 25° relative to the horizontal plane.

[0017] In a conventional manner the drilled production wells 5, 10 are,via upwardly oriented well bores in the areas 16, 20, connected to awell head, from where oil or gas from the formation 1 is supplied to adistribution system on the surface. The well bores 5, 10, 16, 20 areestablished, as is usually the case, by drilling from the surface.

[0018] The drilled production wells 5, 10 may have a longitudinalexpanse of eg about 10,000 ft and preferably extend mutually inparallel, eg at a distance of about 1200 ft. The drilled productionwells 5, 10 may, however, within the scope of the invention, divergeslightly in a direction from the areas 16, 20. The situation shown inFIG. 1 is representative of an authentically occurring course ofdrilling, the scale shown describing distances in ft.

[0019] The invention aims at providing, in the formation, a stress fieldthat ensures that a fracture generated by injection at sufficientlyelevated pressure and rate extends along the well at which the fractureis initiated

[0020] The invention presupposes knowledge of the initial state ofstresses of the formation, ie the state of stresses prior to theup-start of any substantial production or injection. In many cases thestress field in the formation will initially be oriented such that theprincipal stresses are constituted by two horizontal stress componentsand by one vertical stress component. In such cases, determination ofthe initial effective stress field requires determination of fourparameters: σ′_(v) that is the vertical effective stress component,σ′_(H) that is the maximal horizontal effective stress component, andσ′_(h) that is the horizontal effective stress component perpendicularto σ′_(H), and the direction of σ′_(H). The value of σ′_(v) is given bythe weight of the overlaying formation minus the pressure, p, of thepore fluid. The pressure p of the pore fluid can be measured from thewall of a drilled well by means of standard equipment. The weight of theoverlaying formation can be determined eg by drilling through it,calculating the density of the formation along the drilled well on thebasis of measurements taken along the drilled well, and finallydetermining the total weight per area unit by summation. In cases whenσ′_(v) is the larger of the three principal stresses, the determinationof σ′_(h) can be performed eg by hydraulic fracture formation—morespecifically by measuring the stress at which a hydraulically generatedfracture doses. Determination of σ′_(H) can, in cases whenσ′_(v)+ξ(3σ′_(h)−σ′_(H))>3σ′_(h)−σ′_(H), where ξ expresses Poisons ratiofor the formation, for instance be performed by fracturing a verticaldrilled well, where the fracturing pressure will be a function of(σ′_(H)−σ′_(h)) and of σ′_(h). In cases when σ′_(v) is the larger of thethree principal stresses, the direction of σ′_(H) can be determined bymeasuring the orientation of a hydraulically generated fracture thatwill, provided the formation has isotropic strength properties, extendin a vertical plane coincident with σ′_(H). Prior knowledge of the valueof σ′_(H) is not essential if the invention is used to fracture wells ina well pattern that follows the direction of σ′_(H), as is preferred.

[0021] When production is performed in the field, liquids and/or gassesthat flow in the formation will change the state of stresses of theformation. For use in a continuous determination of the state ofstresses in the reservoir, in addition to knowledge of the initial stateof stresses, use may be made of a model calculation of the flow withinthe reservoir as well as a model calculation of the resulting effectivestresses in the reservoir rock. Flow simulation can be performed bystandard simulation software with measurements of production andinjection rates and pressures from the wells as input. From thecalculated stress field, the pressure gradient field can be derivedwhich determines the volume forces by which the solid formation isinfluenced in accordance with the following formula:

b _(x) =−βdp/dx; b _(y) =−βdp/dy; b _(z) =−βdp/dz  1)

[0022] wherein p is the pore pressure within the formation, while β isthe Biot-factor of the formation and x, y and z are axes in a Carthesiansystem of co-ordinates. The effect of these volume forces on theeffective stress field in the formation will follow from the elasticitytheory and may be calculated eg by the method of finite elements.

[0023] By the reference numeral 2, FIG. 1 shows the course of theprincipal stress component σ′_(H) in the formation 1 in the shown planefollowing a production period of six months. As seen, the orientation aof the effective principal stress σ′_(H) relative to the drilledproduction wells 5, 10 is relatively unaffected by the production acertain distance from the production wells 5, 10. In the example, theangle α constitutes about 25°. The designation γ further designates theorientation of σ′_(H) relative to a line indicated by the numeral 15that extends centrally between the drilled production wells 5, 10. Asseen, the angle γ corresponds approximately to the angle α in theexample shown.

[0024] It will also appear that the principal stress component σ′_(H)immediately at the drilled production wells 5, 10 has a modifiedorientation, the principal stress being oriented approximatelyperpendicular to the drilled production wells 5, 10, ie at an angle lessthan the angle β. In other words, the compressive stresses in theformation will, in this area, have a maximal component that is orientedapproximately perpendicular towards the drilled production wells 5, 10.This change of direction is initiated upon onset of production and isdue to the inflow in the drilled production wells 5, 10 of thesurrounding fluids.

[0025]FIG. 2 shows the development of the stresses σ′_(h) and the porepressure p in a cross sectional view through the formation in thesituation shown in FIG. 1 following a production period of six months,the lines 5′, 10′ indicating longitudinally extending vertical planesthat contain the drilled production wells 5, 10.

[0026]FIG. 3 shows how the method according to the invention can beexercised with the object of providing improved operating conditionsfrom the production wells shown in FIG. 1 that will, in the following,be designated by the reference numerals 105, 110. The shown conditionscorrespond to the teachings shown with reference to FIG. 1 inasmuch asthe locations of the drilled production wells 105, 110 are concerned.

[0027] It will appear that, along a line corresponding to the line 15 ofFIG. 1, a further drilled well is produced that extends, in an area 125,from the formation to the surface where it is connected to a pump forthe supply of liquid, preferably sea water, to the drilled well section115. The further drilled well section 115 will, in the following, bedesignated the ‘drilled injection well’.

[0028] Preferably the drilled injection well 115 has the same length asthe drilled production wells 105, 110 and will typically be unlined,meaning that the wall of the drilled well is constituted by the porousmaterial of the formation 1 as such. However, the drilled well 115 canalso be lined.

[0029] Besides, FIG. 3 shows—by means of the curve family 102—the stressrelations in the formation 1 six months following the onset ofproduction. The stress relations reflect that, for a period of time T₁corresponding to the immediately preceding three months, liquid has beensupplied, preferably sea water or formation water, to the formation 1via the drilled injection well 115 and under particular pressureconditions that will be subject to a more detailed discussion below.

[0030] The supply of liquid to the porous formation generallyinvolves—as well known—that the contents of oil or gas in the formation1 between the drilled production wells 105, 110 are, so to speak,displaced laterally towards the drilled production wells 105, 110,whereby the fluids initially in place are produced more quickly. By theinvention the supplied liquid can be caused to give rise to furtherchanges in the state of stresses along the drilled injection well. Asshown in FIG. 3, this can be verified by the angle γ′ between the linedefined by the drilled injection well 115 and the principal stressdirection σ′_(H) being less than the corresponding angle γ for theconditions without supply of liquid by the method according to theinvention, see FIG. 1. This change is detected in the area along theentire drilled injection well. The fact that the orientation of σ′_(H)in the vicinity of the injection well is oriented approximately inparallel with the drilled injection well 115 contributes—as will beexplained in further detail below—positively to achieving the effectintended by the invention. If, as is the case of a preferred embodimentof the invention, it is selected to form the drilled production wells105, 110 and the drilled injection well 115 such that, to the widestextent possible, they follow the orientation 102 of the naturaleffective principal stress σ′_(H) of the formation, it is possible toprovide, at a very early stage following the onset of liquid supply,advantageous conditions for achieving the effect intended with theinvention.

[0031] As will appear from FIG. 4, which illustrates the state ofstresses in the formation 1 in the situation shown in FIG. 3, the valueσ′_(h) in the area at the drilled injection well 115 will, as aconsequence of the supplied liquid, be less than the corresponding valueshown in FIG. 2.

[0032] As mentioned initially, the invention is based on the findingthat, during the supply of liquid to a drilled injection well atelevated injection rates, undesirable fractures may occur that propagatefrom the drilled injection well and into one of the adjoining drilledproduction wells. Study of FIG. 3 will reveal such randomly extendingfracture as outlined by the reference numeral 200. The shown fractureextends vertically out of the plane of the paper, but the fracturemay—depending on conditions prevailing in the formation 1—extend in anyother direction.

[0033] By the invention it is aimed to benefit from the advantages thatare associated with a fracture that extends out of a drilled injectionwell. Study of FIG. 3 will show that by the invention it is, to a largeextent, possible to provide an advantageous fracture in the form of awidely vertical slot that extends along and coincides with the drilledinjection well 115.

[0034] In order to obtain the intended effect in accordance with theinvention, liquid is initially supplied, while production is beingcarried out to the drilled injection well 115 at a relatively lowinjection rate I. This state is maintained as a minimum for a period T₁which will, as mentioned, cause the stress field to be reoriented aroundthe drilled injection well, whereby the numerically smallest normalstress component σ′_(h) is oriented approximately perpendicular to thecourse of the drilled injection well 115. In other words the smalleststress that keeps the formation under compression is oriented towardsthe plane in which it is desired to achieve the fracture. The liquidpressure P in the drilled injection well 115 should, during the periodT₁, be smaller than or equal to the pressure P_(f), the fracturingpressure, that causes tension failure in the formation, and theinjection rate I shall, during the period T₁, be smaller than or equalto the injection rate I_(max) that gives rise to tension failures in theformation.

[0035] Due to the supply of liquid to the drilled injection well 115,local stress changes will occur in the formation along the periphery ofthe drilled injection well, and the invention makes use of this notcheffect at the drilled well 115.

[0036] Above it was described how the flow of fluids changes the stressfield in the reservoir. The resulting stress field can be calculated byadding the stress changes to the initial state of stresses. Inparticular, the stresses can be evaluated along a line in the reservoir,position 115, along which an injector well has been drilled.

[0037] In the above the local variation of the stress field around thewells—caused by the occurrence of a hole in the formation—is notincluded. Within a radius from the drilled well of about three times theradius of the hole, the stress field will depend on the stress fieldevaluated along the line through the reservoir that the drilled wellfollows, but will differ significantly therefrom. The stresses on thesurface of the well bore as such are of particular interest to theinvention, in particular the smallest effective compressive stress—orthe largest tensile stress in case an actual state of tension occurs atthe hole wall. Such stress is in the following designated σ′_(hole,min).In cases where σ′_(hole,min) is a tensile stress, it is counted to benegative, whereas compressive stresses are always counted to bepositive. Calculation of σ′_(hole,min) presupposes in the following thatdeformations in the formation are linearly elastic. Given thiscondition, σ′_(hole,min) can be calculated by a person skilled in theart along a well track with any random orientation relative to anyrandom—but known—state of stresses.

[0038] In cases where a horizontal unlined injector is essentiallyparallel with σ′_(H) (note that production and injection may cause thisparallelism, where it does not apply immediately at the time of drillingof the injector as indicated in FIG. 3), and where σ′_(v), σ′_(H),σ′_(h) are principal stresses calculated along the line in the reservoirwhere the well is drilled, and it further applies thatσ′_(v)>σ′_(H)>σ′_(h), σ′_(hole,min) is to be found on the top and bottomfaces of the hole and is given by the expression:

σ′_(hole,min)=3σ′_(h)−σ′_(v)  2)

[0039] wherein σ′_(h) and σ′_(v) are, in the present context, anexpression of the effective stresses in the formation in the area of theposition of the drilled injection well 115 determined on the basis ofthe elasticity theory with due regard to the ingoing flows, cf. formula1).

[0040] Also, in these cases around the drilled horizontal well,σ_(hole,min) is found along the upper and lower parts of the drilledwell, ie in two regions that are in a horizontal plane as illustrated inFIG. 5. If the drilled well 115 is circular, these areas are locatedwhere the vertical diameter of the circle intersects the circle.

[0041] Since the liquid flow, as mentioned, gives rise to σ′_(h)decreasing over time, σ′_(hole,min) will decrease. It will appear fromformula 2) that σ′_(hole,min, min) decreases when σ′_(v) increases. Theproduction from the drilled production wells 105, 110 gives rise to suchincrease of σ′_(v).

[0042] In order to provide the desired fracture, the injection rate isincreased, as mentioned, after a certain period of time T₁ has elapsedsince the onset of the injection.

[0043] The condition that must be complied with to enable an increase inthe injection rate—and a controlled fracturing of the formation—is inall cases that the relation

σ′_(hole,min)<σ′_(h)  3)

[0044] has been complied with along the part of the well that is usedfor steering the propagation of the fracture.

[0045] Provided the injection rate is increased prior to this conditionbeing complied with, ie before expiry of the requisite period of timeT₁, there will be an increased risk of undesired fractures as describedabove.

[0046] The described course of events is illustrated in FIG. 6 thatshows how the injection of liquid is initiated about 90 days followingonset of production. At a point in time T₁ after onset of injection theabove relation 3) has been complied with. In the example injection isperformed at the injection rate I for further 90 days, at which point intime σ′_(H) has advantageously undergone a considerable change oforientation (γ−γ′) of about 15°. Then the injection rate is increased toa value above I_(max), which is illustrated in FIG. 6 by the pressure inthe drilled injection well increasing. It will appear that σ′_(hole,min)abruptly changes character from compressive stress to tensile stress,whereby the tensile strength of the formation is reached, and fracturingresults.

[0047] It is noted that, in case the injection rate is not increased,according to the theory of the applicant, it is also possible to obtain,in the case shown, the desired fracture when σ′_(hole,min), after agiven period, reaches the value of the tensile strength of theformation. However, in many cases this will cause substantial delays.

[0048] In FIG. 7 a typical measurement result is provided by theso-called ‘step rate’ test for determining the maximally allowableinjection rate I_(max). It is noted that in certain cases, it may berelevant to perform a continuous determination of the maximallyallowable injection rate I_(max). This is due to the fact that I_(max)may vary over time. Thus, during the period of time T₁ it may provenecessary to reduce the injection rate I.

1. A method of controlling the direction of propagation of injectionfractures in a permeable formation (1), from which oil and/or gas isproduced, comprising: that, in the formation (1), a first and a seconddrilled production well (105, 110) are formed next to each other; that,at the drilled production wells (105, 110), a further drilled well (115)is formed that extends between the first and the second drilledproduction well (105, 110); that the production of oil and/or gas isinitiated; that, while oil or gas is being produced, a liquid isconveyed to said further drilled well (115) and out into the formation(1) for a first period of time T₁; characterised in that at least anapproximated determination is performed of the maximally allowableinjection rate I_(max) for the period T₁ in order to avoid fracturingruptures in said further drilled well (115) when liquid is supplied;that the injection rate I for the liquid supplied to the further drilledwell (115) is kept below said maximally allowable injection rate I_(max)for said first period of time T₁; and that the injection rate I isincreased to a value above I_(max) after expiry of the period of time T₁when the relation σ′_(hole,min)<=σ′_(h) has been complied with along thefurther drilled well (115), wherein σ′_(h) is the minimum horizontaleffective stress component and σ′_(hole,min) is the minimum effectivecompressive circumferential stress at the wall of the further drilledwell (115).
 2. A method according to the preceding claim, characterisedin that the drilled wells (105, 110, 115) are established so as to havean essentially horizontal expanse.